THIS INVENTION relates to cobalt catalysts. In particular, the invention relates to a cobalt based Fischer-Tropsch catalyst, to a precursor of such a cobalt catalyst, to a process for preparing a precursor of such a cobalt catalyst, to a process for preparing such a cobalt catalyst, and to a process for producing hydrocarbons using such a cobalt catalyst.
The Applicant is aware of known processes for preparing cobalt based catalyst precursors and which involve slurry phase impregnation of a catalyst support with a cobalt salt, drying of the impregnated catalyst support, and calcination of the dried impregnated catalyst support, to achieve a desired cobalt loading of the support. The resultant precursors are then activated by reduction thereof, to obtain cobalt based Fischer-Tropsch catalysts. These catalysts can display good intrinsic activities when used for Fischer-Tropsch synthesis; however, catalysts having enhanced or superior intrinsic activities cannot readily be obtained using the known processes. It is thus an object of the present invention to provide a cobalt based Fischer-Tropsch catalyst having enhanced initial and/or stabilized intrinsic Fischer-Tropsch synthesis activity, as well as a process for preparing such a catalyst.
According to a first aspect of the invention, there is provided a cobalt based Fischer-Tropsch catalyst, the catalyst including a porous catalyst support and metallic cobalt crystallites within the support, and the catalyst having
i. a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total pre-reduced catalyst mass;
ii. when freshly reduced, a mono-modal Gaussian metallic cobalt crystallite size distribution;
iii. when freshly reduced, a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and
iv. when freshly reduced, a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
It was surprisingly found that the cobalt based catalyst of the first aspect of the invention displayed enhanced initial and stabilized intrinsic Fischer-Tropsch activity.
By xe2x80x98stabilized Fischer-Tropsch synthesis effectiveness factorxe2x80x99 is meant the effectiveness factor of the stabilized catalyst. A stabilized cobalt based Fischer-Tropsch catalyst is defined as a catalyst that has been conditioned completely during slurry phase Fischer-Tropsch synthesis at realistic Fischer-Tropsch synthesis conditions using an ultra pure synthesis gas, ie a synthesis gas that contains no contaminant compounds other than H2 and CO that could affect catalytic deactivation.
By xe2x80x98realistic Fischer-Tropsch synthesis conditionsxe2x80x99 is meant reaction conditions of 225xc2x15xc2x0 C. and 20 bar, and % (H2+CO) conversion of 60xc2x110% using a feed gas comprising about 50 vol % H2, about 25 vol % CO, and with the balance being Ar, N2, CH4 and/or CO2.
By xe2x80x98freshly reducedxe2x80x99 is meant a catalyst that has been activated without subjecting such a catalyst to Fischer-Tropsch synthesis.
In this specification, unless explicitly otherwise stated, where reference is made to catalyst mass, the mass given pertains to the calcined catalyst mass or pre-reduced catalyst mass, ie the catalyst mass before any reduction of the catalyst is effected.
Preferably, the metallic cobalt surface area of the catalyst, when freshly reduced, may be, in m2 per gram of catalyst, from 0.28 xcexa9 to 0.89 xcexa9.
The sizes of a majority of the cobalt crystallites may be greater than 8 nm.
It was surprisingly found that the supported cobalt catalysts of the present invention with their large cobalt crystallites, ie with the majority of metallic cobalt crystallite sizes greater than 8 nm and thus relatively low cobalt metal areas, were not severely affected by oxidation. With these catalysts, while their time zero (ie at the start of a run) intrinsic activities, when used for Fischer-Tropsch synthesis, may be lower than those of supported cobalt catalysts having smaller cobalt crystallites (i.e. having the majority of cobalt crystallite sizes smaller than 8 nm), superior or enhanced initial and stabilized intrinsic activities were surprisingly obtained since there was much less deactivation with the novel catalysts than with supported cobalt catalysts having the majority metallic cobalt crystallite sizes smaller than, or equal to, 8 nm.
The porous catalyst support may be a calcined support. Thus, the supported cobalt catalyst of the first aspect of the invention may be that obtained by impregnating a porous catalyst support with cobalt or a cobalt precursor, particularly cobalt nitrate, calcining the impregnated support to obtain a cobalt catalyst precursor; and reducing the catalyst precursor to obtain the supported cobalt catalyst.
The catalyst support may be a modified catalyst support. The modified catalyst support may comprise catalyst support particles coated with a modifying agent, which may be carbon or one or more metals of Group IA and/or Group IIA of the Periodic Table of Elements, ie one of more metals selected from Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, St, Ba, Ra and mixtures thereof. It was found that the catalyst had a surprisingly large increase in initial intrinsic activity when used for Fischer-Tropsch synthesis, and hence a surprisingly high stabilized intrinsic activity, when a modified catalyst support was used, particularly when a carbon or barium coated support was used.
When barium is used to coat the support, the maximum amount of barium that can be used is determined by the influence of the barium on the pore volume of the support as well as on the solubility of the barium precursor. The pore volume of the support should be large enough to accommodate the required amount of cobalt nitrate to be able to obtain a supported cobalt catalyst with the required cobalt loading. The solubility of the barium precursor should be large enough to be able to add the barium precursor in one impregnation step to the support. In practice the maximum amount of barium may be 10% by mass, based on the catalyst mass. The minimum amount of barium is determined by the minimum amount of barium that is effective in increasing the stabilized intrinsic Fischer-Tropsch activity of the cobalt catalysts, and may be 0.2% by mass.
When carbon is used to coat the support, the maximum amount of carbon that can be used as an effective coating is determined by the influence of the carbon coating on the pore volume of the original catalyst support, as the pore volume of the catalyst support determines how much cobalt can be impregnated into the catalyst support. This is particularly important when a catalyst with a relatively high cobalt loading is required. Similarly, the minimum amount of carbon that can be used as an effective coating is determined by the minimum level of carbon that still provides the required positive effect on the stabilized intrinsic Fischer-Tropsch synthesis performance of the cobalt catalyst. Thus, the maximum level of carbon may be 40 g C/100 g support, preferably 20 g C/100 g support, and more preferably 10 g C/100 g support, while the minimum level of carbon may be 0.1 g C/100 g support, preferably 0.5 g C/100 g support, and more preferably 1.2 g C/100 g support.
In principle, the coating of the catalyst support particles can be effected by any suitable method. For example, the carbon coated catalyst support may be prepared by coating pre-shaped spherical porous catalyst support particles with a uniform carbon based layer in accordance with the method as described in EP 0681868, which is hence incorporated herein by reference.
According to a second aspect of the invention, there is provided a cobalt based catalyst precursor, which includes a porous catalyst support and cobalt oxide crystallites within the support, the precursor having a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total precursor mass, and with the precursor being capable of yielding, on fresh reduction thereof, a catalyst having a mono-modal Gaussian metallic cobalt crystallite size distribution; a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
The porous catalyst support of the precursor may be the same as that of the catalyst of the first aspect of the invention, and may thus be a modified catalyst support as hereinbefore described.
According to a third aspect of the invention, there is provided a process for preparing a cobalt based catalyst precursor, which process comprises
in a support impregnation stage, impregnating a particulate porous modified catalyst support with a cobalt salt, and partially drying the impregnated support, to obtain a partially dried impregnated support; and
in a calcination stage, calcining the partially dried impregnated support to obtain the cobalt based catalyst precursor, with the precursor comprising calcined porous modified catalyst support particles containing cobalt oxide crystallites, and the precursor having a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total precursor mass, and with the precursor being capable of yielding, on fresh reduction thereof, a catalyst having a mono-modal Gaussian metallic cobalt crystallite size distribution; a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
The resultant cobalt catalyst precursor will thus, in practice, be reduced to obtain the cobalt catalyst, which will thus have the enhanced or superior initial as well as stabilized intrinsic activity.
According to a fourth aspect of the invention, there is provided a process for preparing a cobalt based Fischer-Tropsch catalyst, which process comprises
in a support impregnation stage, impregnating a particulate porous modified catalyst support with a cobalt salt, and partially drying the impregnated support, to obtain a partially dried impregnated support;
in a calcination stage, calcining the partially dried impregnated support to obtain a cobalt based catalyst precursor, with the precursor comprising calcined porous modified catalyst support particles containing cobalt oxide crystallites; and
in a reduction stage, reducing the cobalt catalyst precursor, to obtain a cobalt based catalyst which has (i) a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total pre-reduced catalyst mass; (ii) when freshly reduced, a mono-modal Gaussian metallic cobalt crystallite size distribution; (iii) when freshly reduced, a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and (iv) when freshly reduced, a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
The cobalt salt may, in particular, be cobalt nitrate, Co(NO3)2.6H2O.
The modified catalyst support may be any commercially available porous oxidic catalyst support, such as alumina (Al2O3), silica (SiO2), a silica-alumina (SiO2xe2x80x94Al2O3), titania (TiO2) and magnesia (MgO), coated with carbon or one or more metals of Group IA and/or Group II of the Periodic Table of Elements as a modifying agent, as hereinbefore described.
The support may be a protected modified catalyst support, containing, for example, silicon as a modifying component, as described in WO 99/42214 and which is hence incorporated herein by reference.
The processes according to the third and fourth aspects may, if necessary, include preparing the modified support, ie they may include modifying the support particles by coating them with the modifying agent.
The support impregnation may, in principle, be effected by any known impregnation method, eg incipient wetness impregnation, or slurry phase impregnation. Similarly, the calcination may be performed in any known calcination unit, eg fluidized bed, fixed bed, furnace, rotary kiln, and/or torbed calciner, preferably at temperatures between 150xc2x0 C. and 400xc2x0 C., more preferably between 200xc2x0 C. and 300xc2x0 C. In particular, the calcination may be in accordance with that described in U.S. 60/168,604, and which is thus incorporated herein by reference. The calcination may thus involve fluidized bed calcination as described in U.S. 60/168,604.
The cobalt based catalyst precursor may be obtained by a 2-step slurry phase impregnation, drying and calcination process. The 2-step process includes, in a first step, impregnating the catalyst support with the cobalt salt, partially drying the impregnated support, and calcining the partially dried support, to obtain a calcined material, and thereafter, in a second step, impregnating the calcined material with the cobalt salt, partially drying the impregnated material, and calcining the partially dried material, to obtain the catalyst precursor.
The process may include partially reducing the calcined material prior to impregnating it with the cobalt salt. The partial reduction of the calcined material may be effected at a temperature of 100xc2x0 C. to 300xc2x0 C., more preferably at a temperature of 130xc2x0 C. to 250xc2x0 C. The partial reduction of the calcined material may be effected by contacting the calcined material with a hydrogen and/or carbon monoxide containing gas as a reducing gas.
According to a fifth aspect of the invention, there is provided a process for preparing a cobalt catalyst precursor, which process comprises
in a first step, in a support impregnation stage, impregnating a particulate porous catalyst support with a cobalt salt, and partially drying the impregnated support, and, in a calcination stage, calcining the partially dried impregnated support, to obtain a calcined material;
at least partially reducing the calcined material; and thereafter
in a second step, in a support impregnation stage, impregnating the at least partially reduced material with a cobalt salt, and partially drying the impregnated material, and, in a calcination stage, calcining the partially dried impregnated material, to obtain the cobalt catalyst precursor, with the precursor comprising the calcined porous catalyst support with cobalt oxide crystallites present therein, and having a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total precursor mass, and with the precursor being capable of yielding, on fresh reduction thereof, a catalyst having a mono-modal Gaussian metallic cobalt crystallite size distribution; a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
As discussed hereinbefore, the resultant cobalt catalyst precursor will thus, in practice, be reduced to obtain the cobalt catalyst, which will thus have the enhanced or superior initial as well as stabilized intrinsic activity.
Thus, according to a sixth aspect of the invention, there is provided a process for preparing a cobalt catalyst, which process comprises
in a first step, in a support impregnation stage, impregnating a particulate porous catalyst support with a cobalt salt, and partially drying the impregnated support, and, in a calcination stage, calcining the partially dried impregnated support, to obtain a calcined material;
at least partially reducing the calcined material;
in a second step, in a support impregnation stage, impregnating the at least partially reduced material with a cobalt salt, and partially drying the impregnated material, and, in a calcination stage, calcining the partially dried impregnated material, to obtain the cobalt catalyst precursor, with the precursor comprising the calcined porous catalyst support with cobalt oxide crystallites present therein, and having a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total precursor mass, and with the precursor being capable of yielding, on fresh reduction thereof, a catalyst having a mono-modal Gaussian metallic cobalt crystallite size distribution; a metallic cobalt surface area from 0.14 xcexa9 m2 to 1.03 xcexa9 m2, per gram of catalyst; and a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater; and
in a reduction stage, reducing the cobalt catalyst precursor, to obtain the supported cobalt catalyst which (i) has a proportion of its cobalt in reducible form, with this proportion being expressible as xcexa9 mass %, based on the total pre-reduced catalyst mass; (ii) has, when freshly reduced, a mono-modal Gaussian metallic cobalt crystallite size distribution; (iii) has, when freshly reduced, a metallic cobalt surface area, in m2 per gram of catalyst, of from 0.14 xcexa9 to 1.03 xcexa9; and (iv) has, when freshly reduced, a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
The catalyst support may be a modified catalyst support, as hereinbefore described. The support may even be a protected modified catalyst support, as hereinbefore described. The processes according to the fifth and sixth aspects may, if necessary, include preparing the modified support.
The cobalt salt may, in the processes of the fifth and sixth aspects, be cobalt nitrate.
The support impregnation, drying and calcination may, in particular, be in accordance with the process described in our copending WO 00/20116, which is thus incorporated herein by reference. The precursor preparation may thus involve a 2-step slurry phase impregnation, drying and calcination process as described in WO 00/20116, which is dependant on a desired active component loading requirement and the pore volume of the porous oxidic catalyst support.
The support impregnation and drying may typically be effected in a conical vacuum drier with a rotating screw or in a tumbling vacuum drier.
The catalyst precursor may contain between 5 g Co/100 g support and 70 g Co/100 g support, preferably between 20 g Co/100 g support and 50 g Co/100 g support.
During either of the two slurry phase impregnation steps, a water soluble precursor salt of palladium (Pd) or platinum (Pt) or a mixture of such salts may be added, as a dopant capable of enhancing the reducibility of the cobalt. Preferably, the dopant is added in a mass proportion of the palladium metal, the platinum metal or the mixture of palladium and platinum metals to the cobalt metal of between 0.01:100 to 0.3:100.
The invention extends also to a cobalt catalyst, when produced by the process of the fourth or sixth aspect of the invention, and to a cobalt catalyst precursor, when produced by the process of the third or fifth aspect of the invention.
According to a seventh aspect of the invention, there is provided a process for producing hydrocarbons, which includes contacting synthesis gas comprising hydrogen (H2) and carbon monoxide (CO) at an elevated temperature between 180xc2x0 C. and 250xc2x0 C. and an elevated pressure between 10 and 40 bar with a cobalt catalyst according to the invention, in a slurry phase Fischer-Tropsch reaction of the hydrogen with the carbon monoxide, to obtain hydrocarbons.
The invention extends also to hydrocarbons when produced by the process as hereinbefore described.